In the field of circuit design, a bandgap reference circuit is widely used for providing a predetermined voltage. The advantages include low power consumption, stable output voltage, and so on. A bandgap reference circuit usually provides a reference voltage without being affected by temperature by adjusting a voltage across an internal transistor and a current across an internal resistor. Hence, a bandgap reference circuit acts as a voltage source of a voltage regulator.
However, it has to wait a long time for the voltage provided by a present bandgap reference circuit to reach a predetermined value. For example, when a bandgap reference circuit and a linear-drop-out (LDO) regulator are coupled to one another, unwanted noise caused by clock signals generated by the LDO regulator is fed to the bandgap reference circuit. In order to resist and filter out the noise, a larger resistor-capacitor (RC) circuit can be coupled between the bandgap reference circuit and the LDO regulator. Moreover, for decreasing power consumption, a bandgap reference circuit can be designed to have a low leakage current, so the operation current is also low. The foresaid larger RC circuit and the low operation current lengthen the time for waiting an output terminal of the bandgap reference circuit to provide a predetermined voltage. The RC circuit needs more time to be charged. The long waiting time has been an engineering problem in the field.